As tetraethyl lead is phased out, oxygenates have become more important in the petroleum refining industry as a source of gasoline octane boosters. The most common oxygenates for this purpose are the dialkyl ethers, especially those in the C.sub.s to C.sub.7 range. One such dialkyl ether that is generating much interest is diisopropyl ether (DIPE). DIPE is in the boiling range of gasoline, has a high blending octane number, and one reactant generally used in the formation of DIPE, propylene, is a by-product commonly available in refineries. The preparation of DIPE from propylene proceeds by two sequential reactions, where propylene is first hydrated to isopropyl alcohol (IPA) (1) followed by reaction of the alcohol with the olefin (2) or by a single bimolecular dehydration reaction of the alcohol (3) (Williamson synthesis) according to the equations, ##STR1##
These reactions are catalyzed by a variety of catalysts such as activated charcoal, clays, resins, and zeolites. In particular, the reactions may be catalyzed by acidic ion exchange resins including sulfonated cation exchange resins such as sulfonated polystyrene resins and sulfonated styrene/divinylbenzene co-polymers as disclosed in U.S. Ser. No. 08/079,768, G.B. 1,176,620, and U.S. Pat. No. 4,182,914. Halogenated strong acid ion exchange resins such as those described in U.S. Pat. No. 4,705,808, U.S. Pat. No. 4,269,943, and U.S. Pat. No. 3,256,250 also may be used.
A recognized problem of these catalysts is their susceptibility to hydrolysis of the acidic groups causing the transfer of acidic material from the catalysts into the reaction mixture and ultimately into the reactor effluent. The hydrolysis depends strongly on the reaction temperature, and the higher the temperature the greater the degree of hydrolysis. Steps may be taken to remove acid from process streams to protect downstream process units. For example, Neier, W.; Woeliner, J., Hydrocarbon Processing, November 1972, discloses a process of producing IPA from 12.5:1 to 15:1 ratios of water and propylene using a fixed bed of acidic ion exchange resin where reactor effluent is neutralized with sodium hydroxide and the quenching/process water, after being separated from the product IPA, is passed through an ion exchange unit to remove sulfate, sodium, and iron ions before being recycled to the reactor.
Similarly, in U.S. Pat. No. 4,182,914 DIPE is produced from IPA and propylene in a fixed bed containing a strongly acidic ion exchange resin operated at temperatures from 100.degree. to 130.degree. C. The effluent of the fixed bed is passed through an inorganic, particulate acid-neutralizing agent such as magnesium oxides or aluminum oxide prior to being passed to downstream processing units. However, the portion of the effluent that is recycled to the fixed bed is not neutralized prior to recycling with no stated negative effects. Note the absence of water in this DIPE production process since IPA is added in the feedstock and not formed in the fixed bed.
When IPA is formed by olefin hydration in the same reactor where DIPE is formed by etherification of IPA and propylene, the water to olefin mole ratios may range from 0.1:1 to 2:1. Because of the quantity of water present, the reactor is operated at higher temperatures, from 130.degree. to 150.degree. C., thereby increasing the degree of hydrolysis of the acid groups of the acidic ion exchange resin catalyst. U.S. Ser. No. 8/079,768 discloses such a process where the IPA and DIPE are formed in a single-stage reactor and states that the amount of acid may be 10 to 100 times as high for the single-stage process in comparison to processes with very little water and lower operating temperatures. The high concentration of acid, when recycled to the reactor, substantially accelerates the deactivation of the ion exchange resin catalyst. Therefore, U.S. Ser. No. 08/079,768 discloses passing the reactor effluent to an extraction zone to transfer acid from an organic phase to an aqueous phase and then passing the aqueous phase to a base ion exchange unit to remove the acid. After the add is extracted from the organic phase, a portion of the acid-depleted organic phase is recycled to the reactor and a portion is passed to downstream processing units. Note that after extraction, the organic phase is saturated with water. Therefore, the amount of water being recycled to the reactor is not within an operator's control.
Applicants have discovered that through limiting the amount of water in the single-stage reactor to water to olefin mole ratios of from about 0.1:1 to about 0.8:1, the extraction step of U.S. Ser. No. 08/079,768 is not necessary, and the entire reactor effluent may be passed directly to an acid removal zone containing solid particles capable of retaining the acid. Eliminating the extraction step further allows an operator to control the amount of water being recycled to the reactor since the recycle is no longer saturated with water.